Conventional chemical calorimeters have been used to measure temperature changes of more than 20 microdegrees in liquid and solid samples weighing 100 milligrams or more with arrays of thermocouple elements surrounding the sample. To speed the detection of the heat, it is usually conducted as a measured heat flux through arrays of thermoelectric elements whose outputs are integrated and recorded Small samples and small temperature changes are difficult to measure in this way because the thermocouples are brittle semiconductors that are hard to miniaturize.
Thermopiles made of conventional metal wires have been used to study heat changes in nerves and muscles. In these, an array of as many as 100 thermoelectric junctions are pressed against the tissue with thin sheets of mica as electrical and chemical barriers. The thinness of the mica makes the thermopile responses as fast as 10 milliseconds, but the low source impedances and thermoelectric powers of the thermocouples require that the signals be recorded with galvanometers of poor frequency response. Nevertheless, by signal averaging many responses of nerves with such systems, temperature changes of 7-10 microdegrees C. have been seen.
In 1981, I. Tasaki and K. Iwasa (Biochem. Biophys. Res. Commun. 101. 172-176 (1981)) described a new and much more sensitive temperature sensor based on pyroelectric charge displacements in the artificial plastic film polymer, poly(vinylidene 1,1-difluoride). This polymer, PVDF, when coated with aluminum or nickel films and then stretched and heated in a strong electrical field becomes a permanently polarized dielectric material with a very high insulation resistance. When subjected to a temperature change of 1 degree K., PVDF films 9-10 micrometers thick show potential changes between their two surfaces of as much as 2 volts. The voltage response develops fully in the 100 microseconds needed for the full thickness of the film to be heated or cooled, and the signal is quite linear for small temperature changes. Applying a PVDF film to a thin polyester film on which the tissue was placed and leading the PVDF electrical response into the summing point of an operational amplifier, they were able to make a device that could detect 10 microdegree changes in 10 milligram tissue samples in 10 milliseconds. The device was so much more sensitive than thermopiles that signal averaging was usually not needed. Despite its great advantages, Tasaki's and Iwasa's PVDF device was limited in several ways.
(1) It was assembled by bonding together polyester, platinum, and PVDF layers with epoxy resins. Their high viscosity makes it difficult to achieve very thin bonding layers and only one or two devices can be made at a time because of the long setting time of the resin. Also, the detectors are too thick, making their unloaded heat capacity too great and their response times too long.
(2) The pyroelectric charge displacement from the PVDF was converted to a voltage signal by an operational amplifier with a junction field-effect transistor input whose feedback element is a resistor of about 10 gigohms. Because the amplifier must work at high impedance levels and because such large resistors have 1-2 picofarads of shunt capacitance, the high-frequency response of the system is limited. Moreover, the thermal noise generated in the feedback resistor contributes appreciably to the system noise. The resistance of such large resistors is moisture-sensitive and they are usually enclosed in bulky sealed glass tubes to improve their stability.